Controllable route to barium molybdate crystal and their photoluminescence

Article abstract of DOI:10.1016/j.jallcom.2015.03.108

A simple precipitation route has been developed to synthesize BaMoO₄ with different morphologies, such as flower-like spheres, spindle-like and octahedron crystals, by using a novel organic compounds poly-(styrene-alt-maleic acid) (PSMA) as crystal growth modifier. The concentrations of PSMA and reactants have significant influences on the morphology control of BaMoO₄. PSMA efficiently controls the morphology by inhibiting the oriented growth direction. The photoluminescence spectra show broad green emission at 530 nm, which is attributed to the ¹T₂ → ¹A₁ transition in the intrinsic MoO₄^2- group.

Full text of DOI:10.1016/j.jallcom.2015.03.108

Journal of Alloys and Compounds 639 (2015) 102–105
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Controllable route to barium molybdate crystal and their
photoluminescence
a
a
a,
M. Lei a, C.X. Ye a, S.S. Ding a, K. Bi a, H. Xiao a, Z.B. Sun b, , D.Y. Fan , H.J. Yang , Y.G. Wang
⇑
⇑
a State Key Laboratory of Information Photonics and Optical Communications & School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, China
b Center for Space Science and Applied Research, National Space Science Center, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A simple precipitation route has been developed to synthesize BaMoO4 with different morphologies, such
as flower-like spheres, spindle-like and octahedron crystals, by using a novel organic compounds
poly-(styrene-alt-maleic acid) (PSMA) as crystal growth modifier. The concentrations of PSMA and
reactants have significant influences on the morphology control of BaMoO4. PSMA efficiently controls
the morphology by inhibiting the oriented growth direction. The photoluminescence spectra show broad
Received 16 February 2015
Received in revised form 12 March 2015
Accepted 14 March 2015
Available online 19 March 2015
green emission at 530 nm, which is attributed to the 1T2
?
1A1 transition in the intrinsic MoO24À group.
Keywords:
Ó 2015 Elsevier B.V. All rights reserved.
Inorganic materials
Chemical synthesis
Microstructures
Luminescence
1. Introduction
at room temperature. The morphology evolution of crystals is
investigated through adjusting concentrations of both PSMA and
reactants. Room temperature photoluminescence (PL) spectra
show that the optical properties could be modulated by different
morphologies. The biomineralization method is proved to be
convenient, economical, practical and environmental, which may
provide new insights on the controllable synthesis of other
inorganic materials.
In the past few years, intensive efforts have been devoted to the
controllable synthetic method of crystals, since the morphology,
size, structure and dimensionality of crystals have great impacts
on their physical, chemical and other intrinsic properties [1–28].
Among various synthesis methods, biomineralization method
assisted by some special organics has been widely used to synthe-
size crystals with controllable morphology and size [29,30]. A series
of inorganic compounds including CaCO3 [31–33], BaSO4 [34,35],
BaCO3 [36,37] etc., have been successfully synthesized using
effective polymers to control the nucleation and crystal growth
of the as-prepared inorganic compounds. As one of the important
materials in the electro-optical industry, BaMoO4 attracts great
attention due to its promising applications in solid-state lasers
[38], photocatalysts [39] and photoluminescence emission (PL)
[40]. Currently, several methods have been developed to prepare
BaMoO4, such as microwave-assisted synthesis [41,42], hydrother-
mal route [43], microemulsion route [44], complex polymerization
method [45] and electrochemical method [46]. However, biomin-
eralization route to BaMoO4 has been rarely reported to date
[47]. In this work, BaMoO4 crystals with distinct morphologies
2. Experimental
All the chemicals in the experiments were analytical grade and used without
further purification. The polymer, poly-(styrene-alt-maleic acid) (PSMA) (sodium
salt, 13 wt.% aqueous solution), was purchased from Aldrich. The barium chloride
dihydrate (BaCl2Á2H2O) was used as barium source and sodium molybdate dihy-
drate (Na2MoO4Á2H2O) as molybdenum source. In all cases, the concentrations of
both BaCl2 and Na2MoO4 were 0.1 m/L. The typical procedure is described as fol-
lows: designed volume (i.e. 0.25 mL, 1.0 mL and 2.0 mL, respectively) of PSMA
aqueous solution was diluted by 1 L distilled water, 5.0 mL BaCl2 aqueous solution
then were added into 80 mL of the diluted PSMA aqueous solution with vigorous
stirring. Subsequently, exactly same volume aqueous solution of Na2MoO4 was
quickly injected into the above solution. Next, the mixed solution was stirred for
1 min by using a magnetic whisk. During the precipitation process, the reaction
systems were kept at room temperature for 24 h. The final white precipitates were
washed several times with distilled water, then dried at 30 °C in a desiccator for at
least 10 h and stored for further characterization.
were fabricated via
a biomineralization route, by using the
The crystal structures of the as-prepared precipitates were analyzed by X-ray
diffraction (XRD) using the X-ray diffractometer (PANalyticalX’Pert PRO MPD) with
poly-(styrene-alt-maleic acid) (PSMA) as crystal growth modifier
Cu K
a radiation (k = 0.154 nm) in a 2h range of 20–70°. The morphologies of the
as-prepared BaMoO4 were characterized by field emission scanning electron
microscope (FE-SEM, Hitachi S-4800). Room temperature PL spectra were recorded
on a LabRAM Aramis Raman spectrometer with a He–Cd laser (k = 325 nm) as an
excitation source.
⇑
Corresponding authors. Tel./fax: +86 10 62282050.
E-mail addresses: zbsunnssc@sohu.com (Z.B. Sun), wangyg@bupt.edu.cn (Y.G.
Wang).
http://dx.doi.org/10.1016/j.jallcom.2015.03.108
0925-8388/Ó 2015 Elsevier B.V. All rights reserved.

M. Lei et al. / Journal of Alloys and Compounds 639 (2015) 102–105
103
of BaMoO4 changed dramatically as the concentrations of reactants
was increased (sample A1 to A7). At low concentration of reactants,
only a few irregular crystals formed (A1), and then their morphologies
experienced a three-stage transition, the pure flower-like spheres, a
mixed morphology of flower-like spheres and spindle-like crystals,
and the pure spindle-like crystals, respectively. Similar to the phe-
nomena at high concentration of PSMA, the morphologies of
BaMoO4 also underwent such an analogous transition at a moderate
PSMA concentration (PSMA = 1 mL/L, sample B1 to B7). However,
the differences were: (i) the quantity of petals of flower-like spheres
in group B increased and their sizes diminished, (ii) the spindle-like
crystals in group A were changed into big octahedrons with sharp
edges in group B, (iii) the last transition stage, the pure octahedral
crystals disappeared in group B. In contrast to the above
groups, the morphology evolution at low PSMA concentration
(PSMA = 0.25 mL/L, sample C1 to C7) was a gradual and continuous
process from ellipsoids to elongated octahedrons. Furthermore, the
flower-like spheres disappeared. It is important to note that, under
the conditions of extremely low amounts of reactants (sample A1,
B1 and C1, 0.1 mM BaCl2 and 0.1 mM Na2MoO4), only in sample
3. Results and discussion
Fig. 1 shows the typical XRD patterns of the as-prepared pre-
cipitates obtained at different PSMA concentration. All the diffrac-
tion peaks can be readily indexed to the pure scheelite-type
tetragonal structure of BaMoO4 (space group: I41/a), which are in
good agreement with the literature values (JCPDS Card, No. 29-
0193). No additional phase peaks were observed in the resolution
range, indicating that the pure tetragonal phase BaMoO4 can be
obtained at different concentrations of both reactants and PSMA.
The strong and sharp diffraction peaks suggested the synthesized
crystals were well-crystallized. Moreover, the intensities of the
strong peak (112) are almost same in the three samples, indicating
that the PSMA concentration has no effects on the phase structure
and crystallinity.
Fig. 2 summarizes the morphological evolution of BaMoO4 as a
function of concentrations of both reactants and PSMA. When
PSMA = 2 mL/L, i.e. the high concentration of PSMA, the morphology
C1 the regular ellipsoidal crystals with long axis of 1–1.5 lm were
observed. Ignoring the absolute sizes caused by fluctuation effect
and the interaction of the crystallization, the long axis length and
short axis length ratios in group C have a positive correlation with
the concentrations of reactants.
In our previous work, the morphology evolution of BaMoO4
crystals from spheres to octahedrons under the control of poly
(sodium4-styrene-sulfonate) (PSS) were investigated [47]. We
proposed that the PSS controls the morphology and size of crystals
by inhibiting the oriented growth direction. Since the above
experimental result is so consistent with the group C, we suggest
that the controlling mechanism of PSS also can be applied for this
purpose. A schematic formation mechanism of BaMoO4 crystals at
low PSMA concentration is shown in Fig. 3a. As an effective crystal
growth modifier with carboxyl group, PSMA is negatively charged
when it is in the aqueous solution. Based on this reason, PSMA has
a tendency to attract cations or attach on the positively charged
crystal surface. At first, PSMA attracts Ba2+, forming the PSMA–Ba
chain, then the PSMA–Ba chain will attach to the exposed planes
of BaMoO4 crystals. However, different exposed planes of crystals
Fig. 1. XRD patterns of the final precipitates obtained at the following typical
experimental concentrations: (A) 1.0 mL/L PSMA, 0.5 mM BaCl2 and 0.5 mM
Na2MoO4 (B) 2.0 mL/L PSMA, 0.7 mM BaCl2 and 0.7 mM Na2MoO4, (C) 0.25 mL/L
PSMA, 0.3 mM BaCl2 and 0.3 mM Na2MoO4.
Fig. 2. Morphology of BaMoO4 obtained at different concentrations of both reactants and PSMA. ⁄The letter represents the concentration of PSMA, i.e. A = 2 mL/L, B = 1 mL/L
and C = 0.25 mL/L, respectively. The number represents the dosages of the reactants (unit: 0.1 mM). For example, the title ‘‘A5’’ means 2 mL/L PSMA, 0.5 mM BaCl2 and 0.5 mM
Na2MoO4.

104
M. Lei et al. / Journal of Alloys and Compounds 639 (2015) 102–105
Fig. 4. Room temperature PL spectra of BaMoO4 obtained at the following typical
experimental concentrations (a) 1.0 mL/L PSMA, 0.5 mM BaCl2 and 0.5 mM
Na2MoO4 (b) 2.0 mL/L PSMA, 0.7 mM BaCl2 and 0.7 mM Na2MoO4, (c) 0.25 mL/L
PSMA, 0.3 mM BaCl2 and 0.3 mM Na2MoO4.
reduced while their volumes increased as the PSMA/Ba2+ ratio
increases. This may occur because of the well-known Ostwald
Ripening process [49] that the small crystals dissolve first and then
redeposit onto larger crystals to maintain the minimum system
energy. When the local PSMA concentration is high but not enough
to completely inhibit the crystallization of BaMoO4, large particles
usually have lower energy and thus more stable than the small
particles, promoting the Ostwald Ripening process.
Fig. 3. Schematic formation mechanisms of BaMoO4 crystals at different concen-
trations of both PSMA and reactants.
usually have different surface energies, and it is known that crystal
plane with high surface energy grows faster than those with lower
surface energy. PSMA will selectively adsorb on the high-surface-
energy plane, decreasing the local Ba2+ concentration by forming
new PSMA–Ba chain, and inhibiting crystal growth in this direction
by lowering the surface energy. When PSMA is sufficient, the
inhibition in the initially dominant growth directions will be
obvious, causing the morphology of ellipsoid. On the contrary, an
increase in reactants concentration will weaken such inhibition,
explaining the morphology evolution from ellipsoid to elongated
octahedron in group C. These results agree with the analysis of
Yu et al. in that the morphology of crystal is determined by the
adsorptive feature of different PSMA concentration [48]. In
addition, when the local PSMA concentration is much higher than
the Ba2+ concentration, the strong PSMA–Ba complexion will inten-
sively decline the combination of Ba2+ and MoO42À, leading to the
incomplete or irregular crystal morphology (sample A1 and B1).
Fig. 3b shows the schematic evolution mechanisms of the
octahedron or spindle-like to flower-like crystal at moderate or
high PSMA concentration. This transformation process could also
be ascribed to the inhibition of PSMA. Unlike low PSMA concentra-
tion, as PSMA/Ba2+ ratio increases, the surface energy of those
original preferential-growth crystal planes will lower to an extent
that almost identical to that of other crystal planes, thus the crystal
will not only grow along the long axis, but also have the potential
to grow along other direction, achieving the transformation from
one-dimensional growth to multi-dimensional growth. When the
PSMA/Ba2+ ratio is low, the samples will exhibit distinct crystalline
characteristics at different absolute PSMA concentration. At mod-
erate PSMA concentration (Fig. 3b, left), two kinds of crystal mor-
phologies could coexist. It appears that part of the octahedral
nuclei will form the flower-like crystals whereas the remaining
nuclei will grow into big octahedrons. Conversely, local high
PSMA will modulate the growth of nuclei directly, forming the
spindle-like nuclei (Fig. 3b, right). This may explain the sharpness
of the crystal edge and the morphology evolution at two different
PSMA concentrations. It is also found that the number of particles
The optical properties of BaMoO4 nanocrystals were investi-
gated by the photoluminescence (PL) technique. Fig. 4 illustrates
the PL spectra of the typical BaMoO4 samples with excitation
wavelength of 325 nm at room temperature. The samples exhibit
broad green emission peaks of 530 nm, which is consistent with
previous data [26,50,51]. It is well accepted that the emission spec-
tra of metal molybdates are mainly caused by the electronic charge
transfer within the MoO24À unit [27,52]. Previous reports [53,54]
analyze the PL spectra of compounds by the Peakfit deconvolution
program. Based on this method, the PL spectra of BaMoO4 can be
deconvolved into several Gaussian peaks. Using Sample A as an
example, we can get five component curves at 410 nm (8%),
445 nm (13%), 493 nm (16%), 525 nm (48%) and 595 nm (15%),
which represents different types of electronic transitions and
specific atomic arrangements. Abreu et al. [54] attribute the
blue–green emission to an ordered structure with shallow defects,
while the yellow–orange–red emission is caused by a disordered
structure with deep defects. According to the principle of MoO42À
complexes, the coupling between the O-2p (r) and O-2p (p) orbi-
tals and Mo-4d (t2) and Mo4d (e) orbitals are responsible for the
hybridization of molecular orbitals. Among those electron states,
only the 1T2 ? 1A1 transition is allowed [27,55]. We ascribe the
blue and green emission to 1T2 ? 1A1 transition in the intrinsic
MoO24À group (shallow defects) and the orange emission to the
defect MoO3 group (deep defects) [56]. The additional emissions
can be explained by the Jahn–Teller splitting effect [31]. In particu-
lar, our previous study on the photoluminescence of BaMoO4
shows intense blue emission and weak green emission at the same
wavelengths [56], we conjecture this may induced by differences
in the crystal structures. However, accurate explanation for the
PL spectra of BaMoO4 remains to be further explored. There are
many factors that govern the photoluminescence properties of
BaMoO4, such as particle size, crystalline degree, morphology,
and surface defects [55,56]. As is shown in Fig. 4, homogeneous
distribution and small particle size are important factors to
enhance the PL intensity, for crystals with such morphology usu-
ally have fewer dead layers [31,47].

M. Lei et al. / Journal of Alloys and Compounds 639 (2015) 102–105
105
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4. Conclusions
In this study, BaMoO4 crystals with diverse morphologies have
been synthesized by using PSMA as crystal growth modifier via a
biomineralization route. The formation mechanisms of the as-pre-
pared BaMoO4 crystals at different concentrations of both reactants
and PSMA were discussed in detail. PSMA controls the crystal mor-
phology mainly by inhibiting the oriented growth direction, and
higher PSMA concentration will cause the transformation from
one-dimensional growth to multi-dimensional growth. Room tem-
perature photoluminescence (PL) spectra show the maximum
emission peaks at 530 nm, which is ascribed to the 1T2 ? 1A1 tran-
sition in the intrinsic MoO24À group. Homogeneous distribution and
small particle size are important factors to enhance the PL intensity.
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